Microwave drying method and apparatus



1970 H. SCHREIBER ETAL 3,491,457

MICRQWAVE DRYING METHOD AND APPARATUS Filed Oct. 10, 1967 4 Sheets-Sheetl INVENTORS HENRY SHIRE/BER 34 DAV/0 J; GOERZJK.

JEROME W HANK/N A may;

Jan. 27, 1970 H. SCHREIBER ETAL 3,491,457

MICROWAVE DRYING METHOD AND APPARATUS' Filed Oct. 10' 1967 4Sheets-Sheet 2 INVENTORS HENRY SHIRE/Bf? DAV/D J. GOEEZ, JR.

F 1 JEROME yZN AT 0 NEYS MICROWAVE DRYING METHOD AND APPARATUS Filed001:. 10, 1967 4 Sheets-Sheet 5 g INVENTORS \66 may fiiHkf/Bf/P 6 6DAVID J. 606/82, JR.

I EF 7 JEA0ME w HANK/N Jan. 27, 1970 H. SCHREIBER ET AL 3,491,457

MICROWAVE DRYING METHOD AND APPARATUS 4 Sheets-Sheet 4 Filed Oct. 10,1967 ZNW EDAA wm m C6 v m 5 WO mm 5 nDJ I r1 M 1 M 1 n J. m w, W W I a m.04,

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ATTORNEYS FIE--5- United States Patent f 3,491,457 MICROWAVE DRYINGMETHOD AND APPARATUS Henry Schreiber, Port Washington, N.Y., and DavidJ. Goerz, Jr., Menlo Park, and Jerome W. Hankin, Morega, Califi,assignors, by mesne assignments, to Bechtel International Corporation,San Francisco,

Calif., a corporation of Delaware Filed Oct. 10, 1967, Ser. No. 674,293Int. Cl. F26b 5/02, 3/04, 13/02 U.S. Cl. 34-1 Claims ABSTRACT OF THEDISCLOSURE A drying device for a web wherein the solvent to be removedfrom the web is subjected to microwave heating and-high speed gas jets,the gas jets breaking down the solvent boundary layer and carrying otfthe solvent released from the web by the absorption of microwave energy.Apparatus is provided for carrying out the method in an eflicientmanner.

BACKGROUND OF THE INVENTION Dryer for paper and other materialscontaining solvents or moisture wherein microwave energy is combinedwith high velocity gas impingement.

DESCRIPTION OF THE PRIOR ART Heretofore microwave energy has beenemployed for drying paper and similar materials and gas has been used toabsorb and carry away the released solvent. Boundary layer conditionsadjacent to the web being dried act as a high resistance insulator,sharply limiting mass transfer from the web. The low velocity ambientgas is therefore limited in its ability to carry away released solventto the rate at which solvent particles can leave the liquid surface onthe web and cross the boundary layer adjacent to the surface of the web.

In the present invention, microwaves energy is absorbed by the solventin the web. This causes an increase in the thermal energy of the solventby virtue of' directly imparting thermal agitation to its molecules. Ina web of finite thickness or a solvent layer containing solventuniformly distributed throughout it, microwave energy is absorbed by thesolvent throughout the web or solvent layer. The solvent nearer thesurface of the web experiences mass and heat transfer to its ambientsurroundings, thus causing the temperature of the remaining liquidsolvent at the surface to be lower than the remaining liquid solvent inthe interior of the web or solvent layer. Since vapor pressure is afunction of temperature, the vapor pressure of the solvent within theweb or solvent layer will be higher than that at the surface, causing asolvent mass and thermal energy migration to the surface. At thesurface, a high velocity jet of gas is made to impinge upon the surfaceof the web or solvent layer, disrupting the solvent boundary layer atthe surface. Since the boundary layer acts as a resistance to masstransfer away from the surface, mass transfer is enhanced by disruptionof the boundary layer. The solvent molecules so released aresubsequently mechanically entrained and absorbed in the gas stream andexhausted from the apparatus.

SUMMARY OF THE INVENTION Microwave drying resulting from microwaveheating in combination with high velocity gas jets offers a number ofadvantages over other drying methods. For instance, there is threedimensional moisture leveling without the necessity of overdrying, thusimproving product quality. The moisture leveling results from the factthat ICC the wettest areas in the web automatically and instantaneouslyabsorb the greatest amount of microwave energy. It is therefore possibleto achieve a more uniform product and more easily reproducible machineoperating conditions on a fresh start-up. This combination of dryingprinciples requires little space, has a high degree of reliability andis capable of achieving extremely high drying rates. Since surfacecontact of the dryer apparatus with the web is not required to achievethe drying effect, it is possible to set surface coatings on both sidesof a web before any physical contact with the web is made. Bindermigration problems in surface coatings are minimized by this method. Thecombination of microwave energy with high velocity gas impingementresults in a highly efiicient energy transfer means. The systemdescribed in this invention is controllable to a high degree withrespect to all operating parameters, therefore, it is possible tooptimize the combination of microwave energy. Thermal energy andmechanical energy to achieve maximum economy and efficiency.

The use of high velocity gas provides means to sweep moisture from thehigh electric field areas thus permitting the use of high electric fieldgradients with a minimum of RF corona and hence permit the highestpossible field concentration in the web and hence highest possibledrying rate. The process of the present invention is primarily adaptedfor the removal of solvents from continuous flexible webs, such aspartially dried paper, particularly when the moisture content of thepaper has been brought down to about 15' to 20% by conventional dryingmethods known to those skilled in the art and coatings and printingplaced on a substrate. For convenience the invention will be largelydescribed in connection with the drying of paper, although it will beunderstood that the invention is one of broad applicability and can beused for the removal of any polar compound such as water, alcohol,acetone or other solvents from a web, sheet or coating.

In accordance with the present invention, a high velocity gas jet isdirected at the paper in conjunction with the application of microwaveenergy whereby the boundary layer is broken and the solvent moleculeswhich have been driven to the surface of the web by the absorption ofmicrowave energy are forceably removed from the web. By high speed ismeant a velocity of from 10,000 to 20,000 feet per minute. Although theinvention is primarily applicable with air, any gaseous medium can beused such as nitrogen or superheated steam. Thus the invention comprisesthe employment of the combination of microwave heating in conjunctionwith a high speed gas jet disrupting the boundary layer.

In accordance with one embodiment of the invention, a meander line formof waveguide system is employed. However other systems can be used toapply the microwave energy such as an asymmetric stripline orinterdigital slow wave structure which serves as a single sidedmicrowave radiator to distribute the energy to the web.

In accordance with one embodiment of the present invention a splitwaveguide is employed having a substantial separation over at least apart of the waveguide length between these two elements so as to reducethe possibility of arcing due to the accumulation of lint or the like.In accordance with another embodiment of the invention, gas flowsthrough the waveguide to prevent condensation of solvent or theaccumulation of gas borne solids therein.

Preferably the gas is recirculated in the system and only a sufficientpercentage is exhausted to atmosphere to carry away the released solventto prevent condensation in the system. The purpose of this method is tominimize heat loss. This is possible since the gas is not used primarilyas an absorbent but is used primarily as a mechanical means of strippingreleased solvent from the boundary layers and secondarily of conveyingaway the moisture evolved.

In acocrdance with another embodiment of the invention, conductive tabssurround the gas nozzles, which are made of a non-conducting, low losstangent material, the tabs prevent arcing at the close spacing requiredat the air nozzles. Preferably the waveguides which are used to conveythe microwave energy to the paper also form part of the duct system,whereby, gas is carried to and from the web.

In accordance with another embodiment of the invenrected at the movingweb to break up the solvent boundary layer adjacent to the web surfaceand to minimize the carry over of released solvent into adjacentwaveguides.

The invention is primarily adapted for use in the microwave bandsdesignated UHF 9 and SHF 10, i.e. frequencies of about 300 mHz. to30,000 mHz. As a practical matter it is preferred to operate on thefrequencies assigned by the FCC for ISM use at 915 mHz., 2450 mHz., 5800mHz. and 22,125 mHz.

In accordance with one aspect of the invention, an improved power inputmatching structure is provided wherein microwave power is supplied tothe split waveguide structure from one side through a split right anglewaveguide ell with an impedance matching means. Further, the inventionprovides a dummy load at the terminal end of the waveguide so that ifthe web should break, or if all the energy is not absorbed by the weband structure there would be no substantial reflection of power and aconsequent destruction of the equipment but the power is harmlesslyabsorbed in the dummy load.

Other features of the invention are set forth in the balance of thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective, fragmentaryview of a device embodying the present invention.

FIGURE 2 is a diagramatic plan view of a complete drying apparatusembodying the present invention.

FIGURE 3 is an end view of a dryer with some of the parts in section.

FIGURE 4 is a plan view of the dryer with some of the parts in section.

FIGURE 5 is an enlarged section on the line 5-5 of FIGURE 3.

FIGURE 6 is an enlarged section on the line 66 of FIGURE 5.

FIGURE 7 is a section on the line 77 of FIGURE 6.

FIGURE 8 is a perspective view, along the lines of FIGURE 1, showing analternate embodiment of the invention.

FIGURE 9 is a perspective view showing still another embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the description whichfollows it is assumed that a partially dry web containing, for instance,from 15 to 20% moisture is being further dried by the combined action ofmicrowave energy and high speed air jets which are applied alternatelyto the web.

The web 10 is drawn through the dryer by means well known to thoseskilled in the art and not illustrated. This can be in the directionshown by the arrow or in the opposite direction. In the embodimentillustrated, the web is subjected to three successive microwave and airdrying stages although it will be understood that any number of stagesmight be employed. Air under pressure is supplied by a fan 12 whichforces air through duct 14 through R.F. filter 15 to the inlet plenums16. If desired, a heater 18 may be included in the air inlet circuit.Air is exhausted through the outlet pleniums 20 through line 22 whichmay be provided with an exhaust fan 24 to aid in the flow. A return line26 may be provided so that at least a portion of the air can berecirculated to conserve heat. When a return line is employed, dampers28, 29, 30, 31, and 32 are employed in the lines as illustrated toregulate the air flows in the system.

A source of microwave energy 36 is provided which feeds the waveguide,described in detail hereinafter through line 38. The generator 36 can beany well known source of RF energy and in one practical embodiment ofthe invention consisted of a klystron and associated power supplyoperating at 2450 mHz. Suitably the RF generator operates at a frequencyfrom about 300 to 30,000 mHz. Thus the ISM allocated frequencies of 915;2,450; 5,800 and 22,125 mHz. are suitable.

The energy is fed into waveguide generally designated 40. At theterminal end of the waveguide 40 a dummy load 42 is provided to absorbexcess energy and in case of a web break, to absorb total energy input.The waveguide, described hereinafter in detail is split and power isintroduced to the structure through a split microwave tee, one half oftee is on the upper half and one half on the lower half of line 38 as isbest seen in FIGURE 3. The opposite half of the waveguide is providedwith an impedance matching stub 44 tuned with a piston 46 which providesa shorting plane and which is provided with an adjusting screw 48. Atthe outlet end, the previously mentioned dummy load 42 is provided onone half of the waveguide and the dummy load can take the form of awater filled tube 50 set at an angle near the center of the guide.

The opposite terminal half of the waveguide is provided with anadjustable matching stub 52 which can be of the same structure as stub44.

The waveguide proper in the embodiment illustrated in FIGURES 1 through7 consists of two complementary halves so that only One half will bedescribed in detail. The waveguide includes a back wall 54 and sidewalls56 and 58. The side wall 58 can be made as a continuation of the wall 60which divides the inlet and outlet plenums 16 and 20. Thus, the Web 10passes between the two halves of the waveguide and absorbs the microwaveenergy from the electric field within waveguide. After passing throughthe waveguide, the web is subjected to a jet of high speed gas directedthrough the nozzle openings 62. These openings 62 are preferably set atan angle to the plane of the paper as is shown to break up the boundarylayer and arranged so as to minimize moisture entering adjacentwaveguide. These jet openings 62, as is shown in the enlarged views ofFIGURE 6 and 7, may form in a strip of plastic or other suitablenon-conductive low loss tangent material 64 and are supported byconductive straps 66 which electrically connect the walls 20 and 56keeping them at the same potential and thus prevent any possible arcing.This mechanical construction is particularly advantageous since if onedesires to change the size of the nozzle for any reason, it is easy toreplace the small plastic strips 64. Alternately, the jet plate may beconductive and can be formed as an integral part of the structure.

It will be noted that the two halves which together constitute thewaveguide are separated by a substantial distance over at least aportion of the length to minimize the tendency for dust or lint whichaccumulates between the two halves to cause arcing. In the embodimentillustrated they are completely separated. It is preferred that thisseparation be at least inch.

In order to provide some circulation of air through the waveguides andto prevent possible condensation of moisture therein, the back wall 54of the waveguide are provided with a series of small holes 68. These arespaced at intervals along the waveguide, the spacing being at about onehalf waveguide wavelength. These do not contribute substantially to thedrying effect but are only provided to maintain the circulation of gasthrough the waveguide and minimize possiblecondensation of moisturetherein.

In FIGURE 8 there is illustrated another embodiment of the inventionwhich operates on the same general principles but which is slightlydifferent from a mechanical standpoint. Here a gas inlet plenum 70 and agas outlet plenum 72 are provided on each side of the device. Thewaveguides 74 are formed of two halves, as previously described, and gasoutlet tubes 76 are provided at suitable intervals leading from thewaveguide to the outlet plenum 72. The gas jets are formed between onewall of a waveguide and the U-shaped structure 78 which is provided oneach side of the web. Welds 79 are provided at intervals to preventarcing and provide structural strength and dimensional stability. Tubes80 lead from the interior of the members 78 to the outlet plenum 72. Itwill be noted that the jets are of two types, one formed between thewall of the waveguide 74 and the adjacent wall of the -U-shaped member78 as at 82 and also between two adjacent U-shaped members as at 84.Although the principle of operation is generally the same as heretoforedescribed, it should be noted that in this instance, a small amount ofgas is bled out of the waveguide rather than being bled into it aspreviously described.

The devices heretofore described have been symmetrical. Although this isthe preferred configuration, other configurations can be used such asthe asymmetric arrangement shown in FIGURE 9. Here, instead of the usualwaveguide which operates with the sheet in its center, a stripline orinterdigital guide slow wave structure is employed which radiates theweb from one side. This comprises metallic duct 90 which is providedwith the strips or fingers 92 which results in a electromagnetic fioldwith an intensity distribution as approximately shown by the dash line94. Within the guide 90 an inlet plenum 96 is formed by the members 98,the jets 100 being provided by the walls of the U-shaped non-conductivemembers 98. The U-shaped members serve as outlets for the spent air. Atthe opposite side of the web a cover 102 may be provided and this may beof a non-conductive material since it does not serve to confine the RFenergy. An air system is provided on the opposite side by ducts 104.

Although air has been described as the gaseous medium, a gas such asnitrogen or superheated steam might be employed. It is highly importantthat the gas have a high velocity at the jet and must be at least 10,000feet per minute. Velocities of 20,000 feet per minute and up can be usedalthough it is generally uneconomic to go much higher than 20,000 feetper minute. Since the gas is not employed to absorb moisture in theusual sense of the word but is employed to break the boundary layer andto mechanically convey off the moisture, it is not necessary to provideextremely dry gas for carrying out the purposes of the invention, thus asubstantial amount of the air can be recirculated by the damperspreviously shown and described resulting in a great heat economy. It isonly necessary that the gas be sufliciently dry to prevent condensationwithin the various ducts and waveguides.

For application to wide webs, it can be assumed that there will beseveral air distribution modules of the type described along the paperwidth and that multiple power inputs may exist in the direction of paperflow.

Although various preferred embodiments of the invention have been shown,it will be understood that these are for purposes of illustration onlyand are not intended as limitations on the invention which is one ofbroad applicability.

We claim:

1. An apparatus for drying a web comprising in combination:

(a) means for moving a web through said apparatus,

(b) a plurality of conductors of electromagnetic energy across the pathof the web which produces a series of electromagnetic fields in saidweb,

(0) a first gas jet in said apparatus, said jet impinging on said webbetween electromagnetic fields (d) means for supplying gas through saidjet at a velocity of at least 10,000 feet per minute (e) means forsupplying a portion of gas through said conductors whereby a circulationof gas through the conductors is provided to prevent condensation ofmoisture therein and (f) means for supplying energy to said conductorsat a frequency of from 300 mHz to 30 gHZ.

2. The apparatus of claim 1 wherein the wave conductor is a splitwaveguide, with one half of the guide on each side of the web, saidhalves being separated through at least part of their length by at leastA-inch.

3. The apparatus of claim 1 wherein the jets are formed by apertures ina non-conductive material with conduc tive straps spaced at intervalsacross the non-conductive material.

4. The structure of claim 2 wherein each half waveguide forms a portionof a gas conduction system for conveying gas to and from the jets.

5. The structure of claim 2 wherein microwave energy power is suppliedto the split waveguide structure from one said through a split rightangle waveguide ell with an impedance matching means.

6. The structure of claim 2 wherein the waveguide is provided with adummy load at its terminal end.

7. The structure of claim 2 wherein the waveguide crosses the web aplurality of items.

8. The structure of claim 1 wherein the jets are supplied with gas froma pressure source and adjustable means provides whereby part of the gasis recirculated.

9. The structure of claim 7 wherein the gas source includes controlledheating means.

10. The structure of claim 2 wherein gas is admitted to the waveguidethrough holes spacer at /2 waveguide length intervals.

References Cited UNITED STATES PATENTS 2,042,145 5/1936 Darrah 3412,226,871 12/1940 Nicholas 34l XR 2,560,903 7/1951 Stiefel 21910.55

3,237,314 3/1966 Smith 344 3,293,765 12/ 1966 Winkler et a1. 34-1 OTHERREFERENCES 1,014,117 12/1965 Great Britain. 1,463,338 11/1966 France.

KENNETH W. SPRAGUE, Primary Examiner UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION patent NO, 3,491,457 Dated January 27, 1970Inv t HENRY SCHREIBER, et al It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 41; cancel "microwaves" and insert microwave Column 2,line 19; after "energy" insert -combine--.

Column 3, line 11; after "inven-" insert the words tion the gas jets aredi am i 11, cancel "another embodiment of".

Column 3, line 75; cancel "pleniums" and insert --plenums Column 6, line36; change "said" to side Column 6, line 41; cancel "items" and inserttimes Column 6, line 44,- cancel "provides" and insert are providedColumn 6, line 48; change "spacer" to spaced SIGNEB AND Fl 3 .2.5.9

Attach EdwlrdMFlctcher, Ir.

mm I. x m' a a a, S Om flomissioner of Pawn ta-

